My 92 351W would always fluctuate, rapidly moved up to about the "R" then quickly dropped down to full cold, repeat. I took out the thermostat and found it to be failed, installed a new one, now the gauge reads right between "N" and "O" and no longer fluctuates. Heat works great. If your heat isnt so good check your heater core for being clogged, if one hose is hot and the other cold that is probably your issue.
Last edited by Rusty but trusty; 12-04-2013 at 05:13 AM.